This article provides information about the ultrastructure of mitochondria, it’s kinds, associated granules and mitochondrial particles!

A mitochondrion consists of two parts:

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(1) limiting membrane, and

(2) inner mass or matrix Palade, (1952).

1. Limiting membrane:

This membrane is double. Outer membrane is about 60 to 75 A in thickness. Inner membrane is about 50 to 70 A thick and shows many infoldings (plate-like) into the cavity of mitochondrion, which are called mitochondrial crests or cristae. They penetrate the matrix of mitochondrion.

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The cristae (crista singular) greatly extend the surface area exposed to the inside cavity of the mitochondrion, providing ample space for accommodation of enzymes. The cristae usually run at right angles to the long axis of the rod-shaped mitochondrion.

The space between outer and inner membranes is called perimitochondrial space which is 60 to 80 A thick (Tylers, 1973), and it is less dense. It also extends in the crests. Each of these membranes -consists of an outer protein layer, a middle bimolecular layer of lipids and an inner protein layer, i.e., membranes have a trilaminar structure. The outer and inner dense osmophilic layers of proteins are 20 to 25 A thick.

2. Matrix:

It is filled with a relatively dense proteinaceous fluid material which is generally homogeneous, but in some cases it may contain a finely filamentous material or small, highly dense granules of about 40 to 50 A diameter known as intra mitochondrial granules.

Within the matrix are small ribosomes, RNA and one or more molecules of circular DNA, about 4.7 to 5.4 µ long. It has many outgrowths as a result of infoldings of inner membrane. These infoldings are called as cristae. Ribosomes are smaller than cytoplasmic eukaryotic ribosomes and are more nearly the size of prokaryotic ribosomes, measuring about 120-150 A in diameter and transfer RNA.

The characteristic adi-electronic (dense) granules of matrix are 300 A in diameter on an average. These granules are very few or absent in the mitochondria of Protozoa, myocardial cells and resting thyroid cells; whereas in duodenal absorbing cells, liver, pancreas and brush-border cells, о granules are very numerous.

The function of these granules considered to be the sites for binding of divalent cat ions of magnesium and calcium (Ca++ and Mg++), which play an important part in enzymology. The inner membrane and cristae can be seen to be made up of elementary particles (or respiratory assemblies), each composed of a base and a spherical head (8 to 9 mm in diameter) joined by a connecting piece (or stalk). Elementary particles contain enzymes involved specifically in the formation of ATP.

Matrix contains all the soluble enzymes that are involved in the Krebs cycle, in addition to DNA and protein synthesis. The inner membrane carries all the enzymes related to the respiratory chain (ATPase) and to phosphorylation, as well as specific carrier proteins involved in the permeation of metabolites such as ADP, ATP and phosphate. The arrangement of cristae is variable and it may be as follows —

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(1) They may be parallel to the long axis of mitochondria, as in neurons and striated muscle cells.

(2) Commonly they lie perpendicular to the long axis.

(3) These may be in the form of vesicles, often branched to form network of connecting chambers, as in parathyroid gland cells, human leucocytes, some protozoans, etc.

(4) These cristae are also found in a tubular arrangement in adrenal gland cells and has been found in plant meristematic cells and Malpighian tubules of insects.

(5) In certain spermatids the cristae are arranged concentrically inside the matrix.

(6) In Amoeba many cristae become interlaced forming villi.

Variations in the number of cristae is related to the volume of matrix. Where cristae are relatively few (e.g., mammalian liver), there is much matrix, and a large number of cristae (muscle tissue) reduces the amount of matrix.

The number of cristae directly affects the capacity of the mitochondrion to carry on oxidative reactions. In the flight muscle cells of insects, there are more cristae as well as more mitochondria than in any other cells of the body.

[I] Kinds of cristae:

Cristae can be divided into two types: (1) Septate and (2) Tubular.

1. Septate (complete or incomplete) cristae:

These are typical parallel-sided partitions appearing as triple-layered. The partitions (septa) are composed of two unit membranes separated by a continuation of outer chamber. Usually, septate cristae are single and straight; but in frog gastric mucosa and pancreas of bats, they form angulations, i.e., cristae are not arranged in parallel rows but are diverged at 90° angle to others which sometimes produces zig-zag pattern. Related septate cristae are prismatic type, occurring in cricothyroid muscle of bat larynx.

In it, cristae are closely packed, oriented transversely and also fenestrated. They form triangular profiles of 600 A length having prismatic tubules, which appear as parallel strips if cut longitudinally. Finally, septate cristae may form complex partitions besides simple ones. In it, cristae may branch; interconnect to give rise to a honey comb pattern as in human leucocytes.

2. Tubular (villous) cristae:

These appear as villi-like partitions of inner membrane, being found in the mitochondria of protozoans, liver cells and neurons. The terminal part of the tubule may become expanded to form a bulbous area and these cristae have been called saccular or vesicular cristae.

In giant amoeba, Pelomyxa carolinensis, tubular cristae are arranged in a specific manner. At one end of mitochondrion, the cristae are few and irregularly shaped but the other end bears cristae being organized in 3 types of zig-zag pattern.

These infoldings or cristae provide an increased surface area within the mitochondrion for enzymatic activity.

The cristae possess F1 particles and the coupling factors. Further more the activities of several enzymes, (i.e., succinate dehydrogenase, β-hydroxy buty rate dehydrogenase, rotenone sensitive NADH- cytochrome-C-reductase and ATPase) predominate in the cristae.

[II] Associated granules:

In some protozoans and certain fungi, the inner surface of mitochondrial membrane becomes associated with small granules. Pancreas cells of guinea pigs show an increase in the number of these granules with fasting, especially as the mitochondria become associated with lipid droplets. They probably involve in the metabolic activities of mitochondria.

Electron microscopy has revealed the existence of very small particles inside of the inner membrane and the outside of the outer membrane. They were first described by Humberto Fernandez-Moran (1963). The particles of the outer membrane are stalkless, appearing as simple spheres.

These are believed to carry out oxidation reactions which supply electrons and also serve as a site for catalyzing of synthetic reactions with ATP. Each stalked particle of the inner membrane, called elementary or F1 particle, consists of a cuboid base 40 × 110 A, a stem 30 – 40 A in diameter and 45-50 A long, and a round head 75 to 100 A in diameter.

These are closely associated on the surface of a crista and are regularly placed at a distance of 100 A. Depending upon the size and type of mitochondrion, there are from 10,000 to 100,000 elementary particles per mitochondrion.

Functionally the elementary particles are associated with the presence of enzymes for oxidative phosphorylation and with mitochondrial adenosine triphosphatase. Hence these particles are called electron transport particles or ETP (Parsons, 1963). But recently Racker (1967) has shown that F1 particles (ETP) represent a special ATPase or ATP synthetase and take part in the process of oxidative phosphorylation.

The ETP’s catalyze oxidation of NADH and succinate but without coupled phosphorylation (Crane, et al., 1956). Native contains some structural protein and about 30% lipid. It has been separated into four complexes that are the smallest operational units so far isolated from mitochondria —

Is similar to complex I, but catalyzes the oxidation of succinate coupled to reduction of coenzyme Q. It contains 1 mole of flavin (as succinic dehydrogenase), I mole of cytochrome b, lipid, coenzyme Q and nonheme iron.

Complex III:

Catalyzes the oxidation of coenzyme Q coupled to the reduction of cytochrome C. This complex is red in colour and contains two moles of cytochrome b, one mole of cytochrome C1, one mole of nonheme iron protein, and lipid.

Complex IV:

Contains cytochrome a and а3 (cytochrome oxidase) and is green in colour.

These various complexes can recombine with one another in 1: 1: 1: 1 ratio reestablishing the entire chain of electron transfer from NADH or succinate to oxygen. However, since coenzyme Q and cytochrome С are not structurally bound, they must be added back to the preparation for effective oxidation. Green has suggested that each complex carries out coupled phosphorylation.

Respiratory Chain or Electron Transport System (ETS):

Electron transport system consists of a series of complex proteins, which take part in the respiratory chain. There are four complexes formed of lipoproteins and two mobile electron carriers—coenzyme Q (CoQ) or ubiquinone (UQ) and cytochrome C.

[I] Complexes:

Complexes are the sites where hydrogen ions released during kreb’s cycle are oxidized and their energy is trapped in ATP.

According to current view (Hinkle and McCarty, 1978) the head piece of mitochondrial particles contains ATPase proper, the stalk consists of F5 or oligomycin-sensitivity conferring protein (OSCP) and Fe (FC2) and base piece (FO) possesses protein channel. In brief, structure and components of respiratory chain are given below —

1. Complex I:

(NADH—CoQ reductase). It has following components:

(a) NADH dehydrogenase:

It consists of flavoprotein with FMN as prosthetic group. The protein is a single polypeptide chain with m.w. 70,000.

(b) Nonheme iron (NHI):

Proteins with iron-sulfur centres (Fe-S). There are six Fe-S centres, i.e., Fe-SNla, Fe-SNlb, Fe-SN2, Fe-SN3, Fe-SN4 and Fe-SN5. Total m.w. 850,000. It is the – largest complex and includes a flavoprotein containing FMN. This is the first step in the electron transport chain. Electrons are taken into this complex by NAD+ which is located at the matrix side of the membrane.

2. Complex II:

(Succinate—CoQ reductase). It comprises of the following components —

(i) Succinic dehydrogenase with m.w. 70,000 having covalently bound FAD as prosthetic group and two Fe-S centres, i.e., Fe-SSl and Fe-SS2.

(ii) Fe-SS3 protein of m.w. 27,000 and

(iii) Cytochrome b with absorbance 557.5 nm.

Coenzyme Q:

(CoQ) or Ubiquinone (UQ). It is mobile carrier between complexes I and III, and II and III. Complex II precedes the electron transport chain and is coupled to succinate by way of FAD (flavin adenine dinucleotide).

3. Complex III:

(CoQH2-Cyt.C-reductase). This complex contains —

(i) Cytochrome b of m.w. 30,000

(ii) Cytochrome e of m.w. 50,000

(iii) Cytochrome С1 having two polypeptides of m.w. 29,000 and 15,000.

(iv) NH1 protein with FeS centre and m.w. 26,000.

(v) Core proteins

(vi) Antimycin -binding protein.

Cytochrome c:

It is mobile carrier between complexes III and IV with m.w. 13,000. In it is present one с heme bound to polypeptide chain.

4. Complex IV:

(Cytochrome c-oxidase). It contains cytochrome a (Cyt. a) not inhibited by CO, cytochrome аз (Cyt. аз) inhibited by CO and two atoms of copper (Cu and Cu). It is one in which the final oxidation of hydrogen, resulting in water (H2O) takes place.

5. Complex V (ATPase complex):

It contains head piece, stalk and base piece. Head piece (F1) consists of 5 subunits and inhibitor of m.w. 3, 60,000.

Base piece (Fo) is made of proteolipids— a hydrophobic protein complex forming proton channel. There are 4 proteins of m.w. 29,000, 22,000, 12,000 and 7800.

All these complexes and the phosphorylating system are organized within the inner mitochondrial membrane in a highly asymmetrical arrangement. The electron transport system is only accessible to NADH and succinate from matrix side of the membrane, while cytochrome с is reached from cytoplasmic side of the membrane. This molecular organization is consistent with the transfer of protons (H+) across the membrane from matrix side to cytoplasmic side of the membrane.

The respiratory chain is coupled at three points with the system in which phosphorylation of ADP to ATP takes place. The six protons that originated in the respiratory chain are translocated across the inner mitochondrial membrane from matrix side to cytoplasmic side, and these six protons will give rise to three molecules of ATP through the use of mitochondrial ATPase.